Im still waiting for the reveal that the Knights of Ren are somehow evolved from Sabine Wrens clan. Lol.They're not. They've been detailed pretty significantly in the comics. The TL;DR is that they're a dark side cult, used to be big news but haven't been big in a long time. Vader encountered them, and Luke and a young Ben fought them as well. Their leader is Ren, and they hold in high esteem a lightsaber that is also named Ren.